Hydrocarbons (oil, natural gas, etc.) are obtained from a subterranean geologic formation (a “reservoir”) by drilling a well that penetrates the hydrocarbon-bearing formation. In the process of recovering hydrocarbons from subterranean formations, it is common practice to treat a hydrocarbon-bearing formation with a pressurized fluid to provide flow channels, i.e., to fracture the formation, or to use such fluids to control sand to facilitate flow of the hydrocarbons to the wellbore.
Well treatment fluids, particularly those used in fracturing, may comprise water- or oil-based fluid incorporating a thickening agent, normally a polymeric material. Typical polymeric thickening agents for use in such fluids comprise galactomannan gums, such as guar and substituted guars such as hydroxypropyl guar (HPG) and carboxymethylhydroxypropyl guar (CMHPG). Cellulosic polymers such as hydroxyethyl cellulose (HEC) and carboxymethyl cellulose (CMC) may also be used, as well as synthetic polymers such as polyacrylamide. Sometimes the polymeric agent is modified with ionic groups to facilitate hydration of the polymer and to improve crosslinking with metal complexes. Ionic modification of the polymers can reduce the time it takes to dissolve the dry polymer at the well site, and improve both the ultimate gel strength and the thermal persistence of the gel upon crosslinking with a metal crosslinking complex.
In order to prevent the resulting fracture from closing upon release of fluid pressure, a hard particulate material known as a proppant, may be dispersed in the well treatment fluid to be carried into the resulting fracture and deposited therein. The well treatment fluid should possess a fairly high viscosity, such as, a gel-like consistency, at least when it is within the fracture so that the proppant can be carried as far as possible into the resulting fracture. Moreover, it would be desirable that the well treatment fluid exhibit a relatively low viscosity as it is being pumped down the wellbore, and in addition exhibit a relatively high viscosity when it is within the fracture itself. During this process, the proppant is transported by the carrier fluid from the wellbore to the tip of the fracture and does not settle, in which case the treatment and the integrity of the well is jeopardized.
To increase the viscosity, and, therefore, the proppant carrying ability of the fluid, as well as increase its high temperature stability, crosslinking of the polymeric materials may be employed. The viscosity of well treatment fluids (and by extension the transport properties of the proppant) may be enhanced by crosslinking with boron and/or a metal such as chromium aluminum, hafnium, antimony, or a Group 4 metal such as zirconium or titanium. In reference to Periodic Table “Groups,” the new IUPAC numbering scheme for the Periodic Table Groups is used herein as found in Hawley's Condensed Chemical Dictionary, p. 888 (11th ed. 1987). Crosslinking a polymer solution may increase the steady shear viscosity up to two orders of magnitude. For well stimulation treatments, particularly hydraulic fracturing, this creates fracture width and transports proppant. When the treatment is complete, the fluid may degrade, so it can easily flow back to the surface and does not affect the subsequent production of hydrocarbons. Generally, chemicals like oxidizers or enzymes which degrade the polymer backbone are used to drastically reduce the viscosity of the fluid after a few hours under reservoir conditions.
Reversible crosslinking has been adequately described for boron-based crosslinkers, which is considered a non-metal based crosslinker. See U.S. Pat. No. 4,961,466, the disclosure of which is incorporated herein by reference herein in its entirety. However, it appears to be an accepted maxim that that reversibility based upon pH is untrue for metal-based crosslinkers, such as aluminum (Al), titanium (Ti) or zirconium (Zr). As described in the following citation: “To our knowledge, nothing was found on using pH as a triggering agent for metal-based crosslinked gels. It is considered that metal-based crosslinked bonds are irreversible: Unlike borate crosslinker, once the bond between transition metal crosslinker and polymer is broken, it does not reform.” See Gulbis, J.; Hodge, R. M., Fracturing Fluid Chemistry and Proppants. In Reservoir Stimulation Third Edition, Economides, M. J.; Nolte, K. G., Eds. John Wiley and Sons, Inc.: 2000; pp 7-11, the disclosure of which is incorporated by reference herein in its entirety.